The present invention relates to a two-way transmission system for transmitting signals in two directions.
In particular, the invention relates to an insulated type signal transmission circuit for transmitting signals through a transmission line and transformers.
In general, for digital transmission over wires, it is essential that the transmission be of high quality and furthermore it is required that economical use be made of the transmission line. A two-way transmission system using a single line may satisfy these requirements. A frequency division system and a time division system have been well known as techniques for preventing two-way signals from interfering with each other on a single line of a two-way transmission system. However, for the frequency division system, in order to improve transmission capability, it is necessary to correctly and accurately maintain the level difference of transmission signals and the frequency assignment with the result that bandpass filters must be provided which have tight tolerance requirements. Furthermore, since the transmitted signals must be within a predetermined frequency band, it is impossible to transmit baseband signals and it is necessary to employ a suitable modulation system which makes the necessary circuits complex and makes the manufacturing cost quite high.
On the other hand, the time division system has a low efficiency because each signal can be transmitted only in one direction at a time. Furthermore, in each transmission direction, the transmission side has sole control of the line and synchronization between receiving and transmitting sides is difficult. Thus, the time division system is not suitable for high speed transmission.
A double-line type time division two-way transmission system has been proposed in the art in which, as shown in FIG. 1, a master station converts an NRZ (nonreturn-to-zero) double-current signal into an RZ (return-to-zero) signal which is applied to the line L while at a sub-station the RZ signal is received to regenerate the NRZ signal. At the sub-station, a clock signal is extracted from the received RZ signal and subjected to phase shifting so that an NRZ signal from the sub-station is converted into an RZ signal in synchronization with the clock signal thus processed. The RZ signal is transmitted to the master station. Such a system is disclosed, for instance, in Japanese Published Patent Application No. 21963/1978.
More specifically, in FIG. 1, when a signal generator 20 in the master station generates an NRZ digital signal with a predetermined bit width, a signal transmitting and receiving circuit 21 converts the signal into an RZ signal into an RZ pulse with the aid of a clock signal from a clock circuit 22 with the RZ pulse being applied to the transmission line L. The RZ pulse from the sub-station is applied through the signal transmitting and receiving circuit 21 and a transmission suppressing circuit 23 to a recognition circuit 24. In the sub-station, the RZ pulse from the master station is received by a signal transmitting and receiving circuit 25 and is applied through a transmission suppressing circuit 27 to a recognition circuit 29 while it is simultaneously shaped by a pulse shaping circuit 30 and is then applied to a clock circuit 28. The clock circuit 28 generates a clock signal in response to the output of the pulse shaping circuit 30. The clock signal thus generated is applied to a signal generator 26 to control an NRZ digital signal provided by the signal generator 26.
In the system shown in FIG. 1, as the signal transmission of the sub-station is controlled by the clock signal which is provided according to the RZ signal from the master station, the two-way signals never interfere with each other on the transmission line. However, in the system of FIG. 1, it is necessary to convert NRZ signals into RZ double-current signals and to shift the phases of clock signals. Accordingly, it is difficult to simplify the circuitry and to manufacture the system at low cost.
Heretofore, in a signal transmission device as in a computer controlled process input and output system, as shown in FIG. 1, in order to insulate the signal transmitting side from the signal receiving side against differences in the base levels of transmitted and received signals, transformers 4 and 5 are disposed between a master side signal transmitting and receiving circuit 1 and a transmission line 3 and between a slave side signal transmitting and receiving circuit 2 and the transmission line 3.
Recently, it has been required to increase the speed of operation of data processing devices and there has accordingly been a strong demand that a signal transmission device be so improved that it can transmit signals at higher speeds and over longer distance than was previously considered practical.
It can be considered that, for a long transmission line, the circuit parameters, namely, resistance, inductance and capacitance are uniform along the line. Long transmission lines suffer from a problem that, because the cable capacitance is a distributed parameter, it is difficult to increase the rate of signal transmission. This adversely affects the transmission of signals at high rates over long distances.
In view of the foregoing, an object of the invention is to provide a two-way transmission system which provides high transmission quality, stable two-way transmission, excellent anti-noise characteristics, which is implemented with simple circuitry and which is low in manufacturing cost.
Yet further, an object of the invention is to provide an improved insulated type signal transmission system using transformers with a considerably simple circuit arrangement but in which decreases in the signal transmission rate caused by the distributed capacitance of the transmission cable is prevented making it possible to transmit signals at high rate over a long distance and in which a polymorphic arrangement can be provided between master stations and a slave station.